ZrO2 or TiO2 has been heretofore known as a component capable of improving alkali resistance, acid resistance, or mechanical strength of glass used for glass fiber. However, the glass containing a large amount of ZrO2 or TiO2 has considerably high devitrification temperature. So, a devitrified product tends to be deposited during fiber spinning at a bottom of a platinum pot used for spinning, which is called bushing with 200 to 4000 small holes at the bottom. And pulling of glass filaments out of the bushing is interrupted, easily causing thread breakage.
Further, industrial mass production of glass fiber generally must involve spinning at an appropriate temperature for fiber forming, the so-called spinning temperature (temperature at which a melt viscosity becomes 103 poise). At a temperature exceeding a temperature allowing glass fiber forming, that is, at a temperature at which the molten glass viscosity becomes 102.5, glass filaments break and the spinning cannot be carried out. Alternatively, at a temperature lower than the spinning temperature, the glass viscosity increases excessively and glass filaments are hardly pulled out of the bushing.
However, even at the spinning temperature, deposition of a devitrified product at a bottom of the bushing prevents pulling of glass filaments out of the bushing, easily causing thread breakage.
Thus, in order to allow industrial mass production of glass fiber containing a large amount of ZrO2 or TiO2 without deposition of a devitrified product, a devitrification temperature (Ty) of glass must be reduced such that the devitrification temperature does not exceed a spinning temperature (Tx) and a difference between the temperatures (Tx-Ty) is at least 70° C.
Examples of the glass composition containing a large amount of ZrO2 or TiO2, capable of reducing a devitrification temperature of glass and having a difference between the spinning temperature and the devitrification temperature of 70° C. or more, include: glass composition containing alkali metal oxides which are collectively a component for suppressing devitrification of glass (see JP 05-85767 A, for example); and glass composition containing Nb2O5, La2O3, and the like (see JP 10-120438 A, JP 8-25771 A, and Japanese Patent No. 2617632, for example).
Meanwhile, aging of large concrete structures such as bridges has recently progressed and emerged as a problem mainly in cold regions of North America. That is, in the cold regions, when snow, rain, or the like falls on a bridge and the bridge freezes, a large amount of a snow melting agent is used to melt the snow or ice. As a result, reinforcing steel employed as a reinforcement for concrete rusts due to chloride ions in the snow melting agent. Strength of the reinforcing steel as a reinforcement degrades, and the concrete ages rapidly.
It is also pointed in Japan that cracks formed in a concrete wall of a tunnel or the like cause rusting of the reinforcing steel, which may lead to a big accident such as falling off of concrete pieces.
Thus, in civil engineering and construction industries, an alternative material for reinforcing steel accelerating rapid aging of concrete due to rust formation has attracted attention, and glass fiber, FRP rod prepared by hardening a resin with a glass fiber, or the like has been studied as the alternative material. The glass fiber used for such applications is buried in concrete having strong alkalinity, and thus must have alkali resistance and mechanical strength for supporting a large concrete structure.
The glass composition disclosed in JP 05-85767 A has large amounts of ZrO2 and TiO2 for maintaining alkali resistance and mechanical strength of glass, and contains large amounts of alkali metal oxides for suppressing devitrification during spinning. However, it is pointed out that alkali metal ions easily elute from glass because the glass composition contains large amounts of alkali metal oxides, and that a glass structure gradually breaks to cause strength degradation of glass fiber. Further, the glass fiber employed as a reinforcement for an FRP rod has a problem in bonding with a matrix resin. The bonding is weakened due to alkali metal ions eluted from glass, and that a mechanical strength of the FRP rod is degraded.
The glass compositions disclosed in JP 10-120438 A, JP 8-25771 A, and Japanese Patent No. 2617632 each contains large amounts of ZrO2 and TiO2 for improving alkali resistance, and contains Nb2O5 or La2O3 for suppressing devitrification during spinning. However, Nb2O5 or La2O3 is a very expensive component and extensively deforms the network of glass formed of SiO2 and the like. The glass improves in elastic modulus, but becomes brittle and has a significantly degraded tensile strength. Thus, the glass fiber formed of the glass composition containing Nb2O5 or La2O3 has an increased unit material cost and is not suitable as a reinforcement for a large concrete structure under large stress.